The present invention relates to a method of and an apparatus for reducing noise when recording an audio signal by a small-size microphone that is incorporated in a digital consumer electronics device.
Growing efforts have in recent years been made to reduce the size of digital consumer electronics apparatus incorporating a small-size microphone in their cabinet, e.g., video cameras, digital cameras, IC recorders, etc. Because of the small size of those digital consumer electronics apparatus, the user tends to inadvertently touch the microphone or noise is likely to propagate through the cabinet to the microphone when various functional switches are clicked during a recording mode. Therefore, when in a reproducing mode, undesirable touch noise or click noise may possibly be reproduced from the apparatus. Furthermore, since the microphone is positioned closely to a recording device such as a tape device or a disk device housed in the cabinet, vibration noise or sound noise produced by the recording device is highly likely input to the microphone.
In order to reduce the regenerated noise, it has heretofore been attempted to absorb vibrations transmitted from the cabinet and prevent them from being applied to the microphone unit by floatingly supporting the microphone unit of the incorporated microphone with an insulator such as a rubber damper or the like or suspending the microphone unit in the air with a rubber wire or the like. However, these structures are not effective enough to suppress all the vibrations. When strong vibrations are applied or depending on the vibration frequency, the insulator is ineffective or may resonate at an inherent frequency. These proposed structures are difficult to design, and the design difficulty is responsible for obstacles to efforts to reduce the cost and size.
Other noise reduction proposals have also been made (see Patent Documents 1 through 5 below). The noise that is picked up by the microphone unit is caused by not only vibrations transmitted through the cabinet, but also sounds propagated through the air. Since the noise is transmitted through complex paths to the microphone unit, the conventional passive noise reduction techniques are subject to limitations and have not reached a level that the user satisfies.
Patent Document 1: Japanese Patent Laid-open No. 2002-74673;
Patent Document 2: Japanese Patent Laid-open No. 2002-251823;
Patent Document 3: Japanese Patent Laid-open No. Hei 8-124299;
Patent Document 4: Japanese Patent Laid-open No. Hei 7-311903; and
Patent Document 5: Japanese Patent Laid-open No. Hei 8-153365.
The applicant of the present application has proposed noise reduction processes as disclosed in Japanese patent application No. 2002-367234 (Noise reduction apparatus and method) and Japanese patent application No. 2003-285294 (microphone device, noise reduction method, and recording device). According to these prior applications, an adaptive filter is used to generate a pseudo-noise signal, and the pseudo-noise signal is subtracted from an audio signal including noise, thereby reducing the noise.
The adaptive filter that is used tends to require a greater number of taps as the noise signal to be approximated is in a wider frequency band and is continued for a longer time interval. For example, if a noise waveform for a time interval of 10 ms is to be approximated in a frequency band up to the Nyquist frequency at a sampling frequency of 48 kHz, then an adaptive filter having about 480 taps is required.
Since as many product-sum operations as several times the number of taps is needed per sample for processing the data, the overall amount of processing operations is increased, requiring a piece of hardware such as a large logic circuit or a high-speed DSP (Digital Signal Processor). A time delay caused by the processing operations that are required cannot be ignored, resulting in a need for simultaneously delaying the audio signal. Accordingly, desired sounds cannot be recorded in real time.
The present invention has been made in view of the foregoing problems. According to the present invention, the adaptive filter disclosed in the prior applications is not employed, but a human auditory masking effect is utilized to effectively reduce noise through a reduced amount of processing operations without causing any substantial signal delay.
The noise that is to be reduced by the present invention is instantaneous noise caused by vibrations, such as touch noise and click noise referred to above. The vibration noise produced by the recording unit is also instantaneously produced noise such as a seeking sound produced by a magnetic head or an optical pickup in the disk unit, but not noise that is produced at all times by a spindle motor. The differences between the prior art, referred to as Patent Documents 1 through 5, and the present invention will be described below.
Patent Document 1 discloses an audio recording apparatus for recording an audio signal from a microphone while reducing, from the audio signal, noise that is generated when an optical pickup moves over a disk recording medium. Though Patent Document 1 is aimed at solving the same problem as the present invention, it does not utilize a human auditory masking effect according to the present invention.
Patent Document 2 discloses a continuous information recording apparatus for cutting off or reducing noise produced in a seek mode of a disk unit from an audio signal produced by a sound pickup. According to the disclosed continuous information recording apparatus, audio data in a cutoff period is approximately interpolated from signal data prior and subsequent to the cutoff period in order to keep the audio signal continuous. According to the present invention, however, no interpolating circuit is required as no interpolation is performed, and a cutoff period is variable utilizing a human auditory masking effect.
Patent Document 3 discloses an audio recording and reproducing apparatus for reducing noise by replacing audio data in a period containing noise from a movable section with interpolated data that is predicted from audio data prior and subsequent to the period. According to the present invention, however, no interpolating circuit is required as no interpolation is performed.
Patent Document 4 discloses a microphone-contained magnetic recording apparatus for reducing audio signal noise produced when a magnetic head of a camera-combined VTR hits a tape by pre-holding an audio signal in a noise producing period or switching to a signal with a noise band trapped therefrom. According to the present invention, data in a cutoff period does not need to be interpolated as a human auditory masking effect is utilized.
Patent Document 5 discloses a microphone-contained magnetic recording apparatus which reduces audio signal noise produced when a magnetic head of a camera-combined VTR hits a tape only when the audio signal level is lower than a reference level. According to the present invention, a cutoff period is variable utilizing a human auditory masking effect.
The above prior art mainly serves to reduce rotation noise produced from drum-type magnetic recording apparatus and seek noise produced from disk-type recording apparatus. The present invention is additionally aimed at reducing touch noise and click noise because it has a sensor for detecting noise.